Fusion proteins of enveloped viruses play an essential role in the early stages of infection, enabling fusion of the viral envelope with a cellular membrane and release of the internal viral nucleoprotein complexes into cells to initiate the infection process. Although major advances have been made in our knowledge of the structure of viral fusion proteins, many aspects of the fusion process remain unknown. The present application is focused on extending our novel observations concerning the regulation of fusion activity by the cytoplasmic domain of the fusion (F) glycoprotein of a paramyxovirus. In the present application the mechanisms by which the cytoplasmic domain regulates cell fusion, and the consequences for virus-cell interaction, will be investigated. Studies will focus on a recently characterized paramyxovirus, designated SER virus, which is closely related to SV5 but exhibits no observable cell fusion activity in several cell types; this lack of fusion activity is the result of an extended cytoplasmic domain in the SER F protein. To investigate the possible mechanism involved in suppression of fusion activity, the hypothesis will be tested that the extended cytoplasmic domain stabilizes the metastable "pre-fusion" state of the SER F protein, thus increasing the activation energy required for conversion to a fusion-active state. It will be determined if the marked differences in fusion phenotypes between SV5 and SER viruses result in differences in virus-cell interaction, including whether SER virus entry into cells may differ from that of SV5 by involving a low pH-dependent process. It will also be determined whether serial passage of SER virus results in the appearance of fusogenic variants with a truncated cytoplasmic tail, and whether such viruses possess a replicative advantage, The effects of specific changes in the cytoplasmic domain on virus replication will also be investigated in the context of otherwise isogenic recombinant viruses. SER F genes with defined sequence changes will be used to replace the F gene of SV5 in an infectious DNA clone, and their effects on virus replication in cell culture as well as in a mouse model will be determined. The long-term goal is to understand the interactions between the external and cytoplasmic domains which are involved in fusion, as well as the role of the fusion-inhibitory sequences in the biology of the virus.